I would place the auras, walls, and buff beams first. Then switch to an apprentice with either a genie or a dice and place towers. (Don't forget to switch back to your builder pet afterwards.)
Every crystal gets an aura stack, a buff beam, and a mage & spider minion.
1. This is where all the towers should go. Aim one DST at every corner, covering the crystals.
2. This is where you should be tower boosting to hit the middle and south-east crystal, make sure to stand on the railing as I've found it to be more consistent for not dying to ogres or other mobs.